A commonly used type of switch key assembly comprises a frame having a frame column and a plunger slideably mounted on the frame. A biasing or return spring is provided which maintains the plunger in its normal or extended position and which is compressed when the column is momentarily depressed. An actuator is provided which is effective, when the plunger is depressed, to bring about closure of the switch which is associated with the switch key assembly.
It is common practice to use a relatively small diameter coil spring directly beneath the plunger as an actuator. When the plunger is depressed, the coil spring is depressed and moves against parts of the switch which are beneath the coil spring to bring about closing of the switch. The switch contacts may be provided on the opposite surfaces of flexible membranes so that compression of the contact spring causes flexure of one of the membrane towards the other membrane.
The present invention is directed to the achievement of an improved actuator which replaces the coil spring type switch actuator that is commonly used. While coil spring type actuators are satisfactory in many respects, they do have some disadvantages. For example, the springs are manufactured as loose piece items which can become tangled with each other in the assembly process when they are assembled to the frame and the plunger. Furthermore, when the plunger is depressed, the actuator spring has the undesirable effect of increasing the resistance of the plunger to an inordinate extent as the plunger approaches and reaches its fully depressed position. In other words, the contact spring contributes materially and significantly to the degree of force required to depress the plunger particularly at the end of the stroke of the plunger. It would be preferrable if the actuator did not contribute to the force requirements so that the assembly could be designed with more precision and could be designed such that it would have an ideal force travel curve. The amount of force required to depress a plunger is a significant factor in the ease and convenience of operating the key switch.
A switch key assembly in accordance with the invention is of the type comprising a frame having a frame column which has a fixed lower end and a free upper end. A plunger is provided on the frame, the plunger being normally in an extended position and being movable on the column parallel to the axis thereof towards the fixed end of the column to a depressed position. A spring is provided to bias the plunger to the extended position. Normally open switch contacts are disposed proximate to the fixed end of the column, and a switch actuator is provided which is responsive to movement of the plunger and which moves the switch contacts to their closed positions when the plunger is moved to the depressed position. The switch key assembly is characterized in that the switch actuator comprises a bell crank having an actuator arm and a cam follower arm. The bell crank is pivotally mounted on a pivotal axis which is proximate to the fixed end of the column and which extends transversely of the axis of the column. The cam follower arm extends beside the axis of the column and the actuator arm extends transversely of the axis of the column. The actuator arm has an actuator portion which is adjacent to the switch contacts and which is movable towards the switch contacts and closes the switch contacts upon rotary movement of the bell crank in a first direction. The cam follower arm has a first cam follower portion and the plunger has a first cam surface thereon which cooperates with the first cam follower portion to cam the bell crank in the first direction upon movement of the plunger to the depressed position whereby upon movement of the plunger from the extended position to the depressed position, the switch contacts are closed, and upon return of the plunger to the extended position, the bell crank rotates in a second direction, which is opposite to the first direction, and the switch contacts are opened.
In accordance with a further embodiment, the column has an axially extending opening extending therein from the free end towards the fixed end and the plunger is slidably contained in the opening. The plunger has an upper end and a lower end, the lower end being proximate to the fixed end of the column, the upper end being beyond the free end of the column when the plunger is in the extended position. The first cam surface extends from the upper end of the plunger. In accordance with a further embodiment, the upper end of the plunger has a force-receiving extension which extends laterally beyond the free end of the column and the first cam surface extends from the extension.
In accordance with a further embodiment, the assembly has an integral supporting base, the fixed end of the column being integral with the supporting base, and the bell crank being pivotally mounted on the supporting base. The cam follower arm has an end portion which is remote from the pivotal axis and the first cam follower portion is on the end portion. In accordance with a further embodiment, the cam follower arm has a second cam follower portion and the plunger has a second cam surface, the second cam follower portion being between the first cam follower portion and the pivotal axis of the bell crank, the second cam surface and the second cam follower portion being effective, during movement of the plunger from the depressed position to the extended position, to rotate the bell crank in the second direction.